From the past, utility knifes with fixed protruded blades were popular, but recently, improved utility knives with a slide structure where a slider with a blade fixed thereto could be manually slided in back and forth directions by hand, and further could be stored inside the holder body are well known.
In addition to that, products with consideration for safety characteristics exist wherein the slider holding the blade could be slided manually, but when after use, the slider, and thereby the blade, is pulled back inside the holder body.
This type of utility knife is disclosed for example in Japanese Laid-Open Utility Model Publication No. 56-111970 which is a knife where the blade is protruded from the holder body by holding the body by the hand and sliding the slider body against the elastic body by a thumb, and when the cutting operation is finished, the user releases the thumb and the slider is pulled back by the elastic body, storing the blade back inside the holder body. An improved example of such type of utility knife is disclosed in Japanese Laid-Open Patent Publication No. 6-315577 wherein the slider body comprises a main slider and a sub slider, the main slider adjusts briefly the amount of blade to be protruded from the holder body, and the sub slider adjusts in detail said amount of blade protrusion, enabling an exact adjustment of the protrusion of the blade.
However, these products were not very convenient in that the blades could not be fixed in a protruded state from the holder of the knife, and the user had to hold the slider during the whole time of use to keep the protruded state of the blade.
With consideration to the above problem, there is an invention disclosed in Japanese Laid-Open Patent Publication No. 5-212161 wherein the blade could be stopped at the protruded state from the knife body, and when the knife is not used, the blade of the knife could be returned to the protective position by use of a spring.
This invention, however, has safety problems in that the matching teeth mounted on the extending portion of the knife holding body for exchangeably holding the knife and the concave matching portion of the stopper nail supported rotatably on the gripping body of the knife is held together in a stop position by the spring force enforcing the knife holding body in the returning direction, and this stop is released by the centrifugal force operated to the stopper nails. Therefore, the operation of the knife was not stable, and a swinging movement of the knife itself to occur centrifugal force was dangerous.